Method of controlling an epitaxial growth process in an epitaxial reactor

ABSTRACT

A method of controlling an epitaxial growth process in an epitaxial reactor and a system for controlling an epitaxial growth process in an epitaxial reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. 119 and37 C.F.R. 1.55 to the prior-filed foreign application, GermanApplication No. 10 2007 017 592.4, filed Apr. 13, 2007, titled “Methodof Controlling An Epitaxial Growth Process In An Epitaxial Reactor”, theentirety of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to a method of controlling an epitaxial growthprocess in an epitaxial reactor. More particularly, the inventionrelates to a closed-loop multivariate controller for a silicon-germaniumcompound epitaxial growth reactor.

SUMMARY

In a silicon-germanium (SiGe) epitaxial reactor, four essential processparameters—silicon growth rate, silicon germanium growth rate, germaniumconcentration and dopant concentration—have to be controlled for averageand also for uniformity. The main influence on these process parametersis the long term deviation of temperature readings due to the ageing ofthe thermocouples that are used to measure the reactor temperature. Thismakes the epitaxial process difficult to control such that the epitaxialstructure obtained is not within specification.

It is an object of the present invention to achieve improved processcontrol in an epitaxial reactor.

With the foregoing in mind, the present invention provides a method ofcontrolling an epitaxial growth process in an epitaxial reactor. Themethod comprises performing a first run of the epitaxial growth process,and controlling the temperature of the epitaxial growth process duringthe first run by a temperature measuring device. The temperaturemeasuring device determines the temperature based on a firstthermocouple offset parameter value. The thermocouple offset parametervalue is optimized for a second run by measuring an actual outputparameter value of the growth process of the first run, setting up amodeled output parameter value by use of the actual output parametervalue and a second thermocouple offset parameter value, determining adistance between a target output parameter value and the modeled outputparameter value, and determining the second thermocouple offsetparameter value as the value of thermocouple offset parameter whichprovides the minimum distance between the modeled output parameter valueand a target parameter value. A second run of the epitaxial growthprocess is performed and the temperature of the epitaxial growth processis controlled during the second run by the temperature measuring deviceusing the determined second thermocouple offset parameter value fortemperature measurement. The first and second thermocouple offsetparameters can also be a set of thermocouple parameter values ratherthan a single value. Primarily, the method of the present invention isbased on the recognition that aging of the thermocouples of atemperature measuring device must be considered carefully during anepitaxial growth process. Accordingly, in a closed loop controlmechanism, the relevant thermocouple parameters are continuously andautomatically adapted without changing the process parameters. Thegeneral process parameters can be fully maintained. Maintaining theprocess parameters reduces the risk of introducing errors in a wellestablished process. The invention compensates for process drift that isdue to thermocouple aging by employing a closed-loop controller thatdownload recalculated thermocouple offsets to the epitaxial reactorbefore each process run. As a result, process parameters—such as reactortemperature, gas flows, and process time—are not adjusted (only thethermocouple offsets which are caused by thermocouple aging areadjusted). Although the thermocouple parameters for each run are notprecisely determined, the method according to the invention provides avery good estimate of the actual values of the thermocouple offsetsduring the next run. Accordingly, the present invention allows thethermocouple values to be anticipated continuously from a preceding runto a following run of the process and allows the thermocouple offsets tobe compensated for, in particular the change of the thermocoupleoffsets.

Preferably the actual output parameter value is averaged over aplurality of process runs before being used for determining the modeloutput parameter value and can be an exponential weighted mean average(EWMA) of process parameters output from the epitaxial reactor over nprocess runs in the epitaxial reactor. This means that the analysismodel is less sensitive to data noise because it is derived fromhistorical data over all runs. Therefore a more accurate value for thethermocouple offset can be obtained.

The parameters can be silicon growth rate, silicon germanium growthrate, germanium concentration and/or dopant concentration, which dependon temperature. The measurement of the output parameters (Si thicknessor growth rate, Ge thickness or growth rate, Ge concentration and dopantconcentration) is carried out with X-ray diffraction spectrometry and a4-point measurement on processed SiGe wafers. The method of the presentinvention can thus be used to control all of the important processparameters in an epitaxial growth process.

The present invention also provides a controller for controlling processparameters in an epitaxial reactor. The controller comprises a means todetermine a second set of thermocouple offset values using an analysismodel, which is based on a difference between a target output parametervalue and a modeled output parameter value. The modeled output parametervalue is determined by use of the actual output parameter value and thesecond set of thermocouple offset values, such that the second set ofthermocouple offset values provides a minimum distance between themodeled output parameter value and the target parameter value.

One thermocouple offset parameter value of the second set ofthermocouple offset values can relate to a central region of the reactorand another thermocouple offset value can relate to a peripheral regionof the reactor so that the temperature can be measured at both thecentre and the edge of the epitaxial reactor.

The present invention provides a data analysis module including ananalysis model for modeling an actual parameter of an epitaxial growthprocess in terms of a temperature measuring device offset and a processrun number so as to obtain a solution for the device offset thatminimizes a difference between the actual parameter and a targetparameter for an epitaxial growth process.

Thus the present invention provides the advantage of a better processcontrol, since offsets in the measurement device are compensated for.Furthermore, process engineering time is significantly reduced since themeasurement device offsets are automatically adjusted in a closed-loopmanner after each process run. Also, since no further hardware isrequired to be installed in the epitaxial reactor, additional processcosts are minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and characteristics of the invention ensue from thedescription below of a preferred embodiment and from the accompanyingdrawings, in which:

FIG. 1 is a three dimensional plot of the distance of the actual processparameters from the target values against thermocouple offsets.

FIG. 2 is a contour plot of the distance of the actual processparameters from the target values against thermocouple offsets.

FIG. 3 is a schematic diagram of a system that includes a controller forcontrolling process parameters in an epitaxial reactor according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In an epitaxial reactor employed to grow silicon-germanium layers, fourdifferent process parameters must be controlled at the same time inorder to get the right compound characteristics. These parameters arethe silicon growth rate, the silicon germanium growth rate, thegermanium concentration and the dopant concentration. These processparameters have to be controlled for average as well as for uniformity.

Once the epitaxial process set-up has been optimized in terms oftemperature, gas flows and time, it has been found that the main processdisturbance is mainly due to long term deviation of temperature readingscaused by thermocouples aging.

Two thermocouples are employed in an epitaxial reactor—one to measurethe temperature in the centre of the epitaxial reactor c and one tomeasure the temperature at the edge of the reactor e.

The offset parameter values for the thermocouples c and e caused byaging of the thermocouples can be defined by a two-dimensional vector dthat has components dc and de.

The four process output parameters—silicon growth rate, silicongermanium growth rate, germanium concentration and dopant concentrationcan then be defined by a vector Y_(i). The vector Y_(i) can be modeledfor the epitaxial process run number n in terms of d as follows:

Y _(i)=γ_(0ni) *[u+ 1/00(Y _(i) d)](with i=1,2,3,4)  (1)

where:

u is a two dimensional vector of components(1,1)  (2)

Y _(i) is 2×2 array of coefficients Y _(i)(k,m)(k=c,e m=c,e)  (3)

The Y_(i)(k,m) coefficients are experimentally determined and describethe sensitivity of outputs Y_(i) to the d changes.

γ_(0ni) is a two dimension vector determined by an exponential weightedaverage (EWMA) of the historical data over all the n process runs. Ifthe actual value for the output Yi at the run number=n is Yia_(n), thenthe γ_(0ni) value for the same run is determined as:

γ_(0ni) =k Yia _(n)+(1−k)γ_(0(n−1)i)(with 0<=k<=1)  (4)

where

γ_(00i)=Yia₀  (5)

The EWMA reduces the model sensitivity to data noise.

For each output, a target value T_(i) to be achieved is determined (theactual value of the process parameter(s) required to produce the desiredepitaxial structure), where T_(i) is a two dimensional vector ofcomponents (T_(ic), T_(ie)).

Replacing Y_(i) with T_(i) in Equation (1) and inverting it, it ispossible to determine the values of the independent variable d.

Four different solutions are found for the vector d, one for each modelY_(i).

A best compromise solution for the vector d can then be found, which isthe minimum of the predicted distance of outputs Y_(i) from all the fourtargets T_(i).

The distance of the output vector Y_(i) from target vector T_(i) is:

D _(i) =Sqrt[(Y _(ic) −T _(ic))^(̂2)+(Y _(ic) −T _(ic))^(̂2)]  (6)

Replacing Y_(i) with the model equation (1) we obtain:

$\begin{matrix}{D_{i} = {{Sqrt}\left\{ \begin{bmatrix}{{_{0\; {nc}}*\left\lbrack {1 + {{1/100}\left( {{{Y_{i}\left( {c,c} \right)}d\; c} + {Y_{i}\left( {c,e} \right)}} \right\rbrack} - T_{ic}} \right\rbrack^{\hat{}2}} +} \\\left( {{_{0\; {ne}}*\left\lbrack {1 + {{1/100}\left( {{{Y_{i}\left( {e,c} \right)}d\; c} + {{Y_{i}\left( {e,e} \right)}{de}}} \right)}} \right\rbrack} - T_{ie}} \right)^{\hat{}2}\end{bmatrix} \right.}} & (7)\end{matrix}$

In order to compare the distances among them, equation (7) is normalizedto the target, as follows:

D _(i) −D _(i) /IT _(i) I=D _(i) /Sqrt[T _(ic) ^(̂2) +T _(ie) ^(̂2)]  (8)

The overall normalized distance from all four targets is the sum of alldistances D ₈, as follows

D=Σ_(i) D _(i)  (9)

It can be seen that Y_(i) is dependent on the thermocouple offsetparameters defined by the vector d. The distance of the vector Y_(i),representing the actual output values of the process output parametersfrom the vector T_(i), representing the target output parameter valuesof the process parameters (the uniform process parameters needed to growthe required Si—Ge layer structure) is then D.

Since the adopted models Y_(i) for the process output parameters arelinear relationships versus d, the D function has one absolute minimum.This can be seen from FIG. 1 (showing the overall % deviation from thetarget in 3D) and FIG. 2 (showing the distance of the actual processparameters from the target values); both of which show the distance Dagainst the thermocouple offsets de and dc. Therefore, the thermocoupleoffset parameter values can be determined from the model. It is thenpossible to adjust the thermocouples by an amount equal to thecalculated offsets to compensate for the actual thermocouple offsetparameter values and thus achieve (or approximate) the target outputparameter values of the process parameters, based on the above model.

One of these linear models is provided for modeling each of the fourprocess output parameters: the silicon growth rate, the silicongermanium growth rate, the germanium concentration, and the dopantconcentration. Using these four models it is possible to relate thevalues of two independent process variables (the thermocouple offsetparameters), the actual process parameter output values and the targetoutput parameter values to achieve the optimum process parameters. Theactual output variables are sent from the measurement equipmentautomatically to the analysis model. Thus the epitaxial reactor isautomatically tuned for each process run.

This multivariate analysis for calculating the (variable) thermocoupleparameter values in order to achieve the minimum distance from all eighttargets is accomplished by a Downhill Simplex algorithm embedded in aclosed-loop controller (discussed below). The process model andmultivariate optimizer are joined in the closed-loop controller that isable to calculate the process set-up in order to achieve the targetprocess. The process setup data is sent to the epitaxial reactor beforeeach run and the next process is executed with the optimizedthermocouple offset parameter values.

FIG. 3 shows a controller 1 according to an embodiment of the inventionthat has an exponential weighted mean average (EWMA) filter 2, aprocessor 3 and an optimizer 4. The processor 3 is provided with theabove-described models Y_(i). The output of the optimizer 4 is connectedto an epitaxial reactor 5. The epitaxial reactor 5 is equipped with anedge thermocouple e for measuring the process temperature at the edge ofthe reactor 5 and a center thermocouple c for measuring the temperaturein the centre of the reactor 5 (the thermocouples c and e are not shownhere). The outputs of the thermocouples c and e, inside the reactor 5are connected to a data analysis module 6, which determines the actualvalues of the process parameters for a particular growth processaccording to the measurements obtained by the measuring apparatus.

In operation, as silicon-germanium layers are grown inside the epitaxialreactor 5, the output parameters obtained from the epitaxial reactor 5by the measuring apparatus are input to the data analysis module 6,which outputs the actual process parameters to the exponential weightedmean average (EWMA) filter 2. The EWMA filter 2 determines the vectorY_(i) for the four process parameters over a number of process runs n:silicon growth rate, silicon germanium growth rate, germaniumconcentration, and dopant concentration. The four different solutionsfor the vector d (the value of d that minimizes the distance D) are thenfound for each model Y_(i) by the processor 3. Based on the solutionsfor the vector d, the thermocouple offset parameter values for thethermocouples c and e are determined. The optimizer 4 then adjusts thesetting of the thermocouples c and e inside the epitaxial reactor 5based on the calculated thermocouple offset parameter values, by anamount that minimizes the difference between the actual process outputparameters and the required (target) process output parameters. Thismeans that the thermocouple offset values are optimized for the nextprocess run.

The process model in the processor 3 and the multivariate optimizer 4are therefore joined in a closed loop controller 1 that calculates theprocess set-up on a process run-by-process run basis in order to achievethe target process, i.e. in order to achieve the target output parametervalues. The process set up data from one process run is sent from thecontroller 1 to the epitaxial reactor 5 and the next process run will beexecuted with the optimized values (thermocouple offset parametervalues).

Thus the controller of the present invention achieves run-to-runcompensation of a change in the thermocouple offset parameter values,which leads to an improved process control, without any further processset-up adjustment.

Although the present invention has been described with reference to aspecific embodiment, it is not limited to this embodiment and no doubtfurther alternatives will occur to the skilled person that lie withinthe scope of the invention as claimed.

1. A method of controlling an epitaxial growth process in an epitaxialreactor, the method comprising: performing a first run of the epitaxialgrowth process, and controlling a temperature of the first run of theepitaxial growth process with a temperature measuring device thatdetermines the temperature based on a first thermocouple offsetparameter value; optimizing the thermocouple offset parameter for asecond run by the steps of: measuring an actual output parameter valueof the epitaxial growth process of the first run; setting up a modeledoutput parameter value as a function of the actual output parametervalue, and a second thermocouple offset parameter value; determining adistance between a target output parameter value and the modeled outputparameter value; determining the second thermocouple offset parametervalue as a thermocouple offset parameter value that provides the minimumdistance between the modeled output parameter value and the targetparameter value; and performing a second run of the epitaxial growthprocess, and controlling a temperature of the epitaxial growth processduring the second run by a temperature measuring device that determinesthe temperature based on the second thermocouple offset parameter value.2. The method according to claim 1, wherein the actual output parametervalue is averaged over a plurality of process runs before being used forthe setting up of the model output parameter value.
 3. The methodaccording to claim 2, wherein the actual output parameter value is anexponentially weighted mean average of actual output parameter valuesthat are output from the epitaxial reactor over n process runs in theepitaxial reactor.
 4. The method according to claim 1, wherein theactual output parameter value is a silicon growth rate, a silicongermanium growth rate, a germanium concentration, or a dopantconcentration.
 5. A system for controlling an epitaxial growth processin an epitaxial reactor, the system comprising: an epitaxial reactor forperforming a first run of the epitaxial growth process, and controllinga temperature of the epitaxial growth process during the first run by atemperature measuring device that determines the temperature based on afirst set of thermocouple offset parameter values; and a controller foroptimizing a second set of thermocouple offset parameters for a secondrun comprising: a EWMA filter for receiving an actual output parametervalue of a growth process of the first run that is output from a dataanalysis module; a processor for setting up a modeled output parametervalue as a function of the actual output parameter value and a secondset of thermocouple offset parameter values, the processor alsodetermining a distance between a target output parameter value and themodeled output parameter value; an optimizer for determining the secondset of thermocouple offset parameter values as a set of thermocoupleoffset values that provides the minimum distance between the modeledoutput parameter value and the target parameter value; wherein theepitaxial reactor also performs a second run of the epitaxial growthprocess, controlling a temperature of the epitaxial growth processduring the second run by a temperature measuring device that measurestemperature using the determined second set of thermocouple offsetparameter values from the optimizer.
 6. The system according to claim 5,wherein a first thermocouple offset value of the second set ofthermocouple offset parameter values relates to a central region of thereactor and a second thermocouple offset value relates to a peripheralregion of the epitaxial reactor.
 7. The system according to claim 5,wherein the actual output parameter value is a silicon growth rate, asilicon germanium growth rate, a germanium concentration, or a dopantconcentration.